Small Pore Molecular Sieve Supported Copper Catalysts Durable Against Lean/Rich Aging for the Reduction of Nitrogen Oxides

ABSTRACT

A method of using a catalyst comprises exposing a catalyst to at least one reactant in a chemical process. The catalyst comprises copper and a small pore molecular sieve having a maximum ring size of eight tetrahedral atoms. The chemical process undergoes at least one period of exposure to a reducing atmosphere. The catalyst has an initial activity and the catalyst has a final activity after the at least one period of exposure to the reducing atmosphere. The final activity is within 30% of the initial activity at a temperature between 200 and 500° C.

FIELD OF THE INVENTION

The present invention relates to small pore molecular sieve supported copper catalysts that are durable after being exposed to a reducing atmosphere, particularly after high temperature exposure.

BACKGROUND OF THE INVENTION

Selective catalytic reduction (SCR) of NO_(x) by nitrogenous compounds, such as ammonia or urea, has developed for numerous applications including for treating industrial stationary applications, thermal power plants, gas turbines, coal-fired power plants, plant and refinery heaters and boilers in the chemical processing industry, furnaces, coke ovens, municipal waste plants and incinerators, and a number of vehicular (mobile) applications, e.g., for treating diesel exhaust gas.

Several chemical reactions occur in an NH₃ SCR system, all of which represent desirable reactions that reduce NO_(x) to nitrogen. The dominant reaction is represented by reaction (1).

4NO+4NH₃+O₂→4N₂+6H₂O  (1)

Competing, non-selective reactions with oxygen can produce secondary emissions or may unproductively consume ammonia. One such non-selective reaction is the complete oxidation of ammonia, shown in reaction (2).

4NH₃+50₂→4NO+6H₂O  (2)

Also, side reactions may lead to undesirable products such as N₂O, as represented by reaction (3).

4NH₃+4NO+3O₂→4N₃O+6H₂O  (3)

Catalysts for SCR of NO_(x) with NH₃ may include, for example, aluminosilicate molecular sieves. One application is to control NO_(x) emissions from vehicular diesel engines, with the reductant obtainable from an ammonia precursor such as urea or by injecting ammonia per se. To promote the catalytic activity, transition metals may be incorporated into the aluminosilicate molecular sieves. The most commonly tested transition metal molecular sieves are Cu/ZSM-5, Cu/Beta, Fe/ZSM-5 and Fe/Beta because they have a relatively wide temperature activity window. In general, however, Cu-based molecular sieve catalysts show better low temperature NO_(x) reduction activity than Fe-based molecular sieve catalysts.

In use, ZSM-5 and Beta molecular sieves have a number of drawbacks. They are susceptible to dealumination during high temperature hydrothermal ageing resulting in a loss of acidity, especially with Cu/Beta and Cu/ZSM-5 catalysts. Both beta- and ZSM-5-based catalysts are also affected by hydrocarbons which become adsorbed on the catalysts at relatively low temperatures and are oxidized as the temperature of the catalytic system is raised, generating a significant exotherm, which can thermally damage the catalyst. This problem is particularly acute in vehicular diesel applications where significant quantities of hydrocarbon can be adsorbed on the catalyst during cold-start; and Beta and ZSM-5 molecular sieves are also prone to coking by hydrocarbons.

In general, Cu-based molecular sieve catalysts are less thermally durable, and produce higher levels of N₂O than Fe-based molecular sieve catalysts. However, they have a desirable advantage in that they slip less ammonia in use compared with a corresponding Fe-molecular sieve catalyst.

WO 2008/132452 discloses a method of converting nitrogen oxides in a gas to nitrogen by contacting the nitrogen oxides with a nitrogenous reducing agent in the presence of a zeolite catalyst containing at least one transition metal, wherein the wherein the at least one transition metal is selected from the group consisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Ir and Pt.

WO 2008/106518 discloses a combination of a fiber matrix wall flow filter and a hydrophobic chabazite molecular sieve as a SCR catalyst on the fiber matrix wall flow filter. The filter purportedly achieves improved flexibility in system configuration and lower fuel costs for active regeneration. Such active regeneration would likely encompass exposure to lean atmospheric conditions. The reference, however, does not contemplate subjecting the filter to reducing conditions. The reference also fails to disclose or appreciate maintaining the durability of a catalyst after being exposed to such a reducing atmosphere.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a method of using a catalyst comprises exposing a catalyst to at least one reactant in a chemical process. The catalyst comprises copper and a small pore molecular sieve having a maximum ring size of eight tetrahedral atoms. The chemical process undergoes at least one period of exposure to a reducing atmosphere. The catalyst has an initial activity and the catalyst has a final activity after the at least one period of exposure to the reducing atmosphere. The final activity is within 30% of the initial activity at a temperature between 200 and 500° C.

According to another embodiment of the present invention, a method of using a catalyst comprises exposing a catalyst to at least one reactant comprising nitrogen oxides in a chemical process comprising exhaust gas treatment. The catalyst comprises copper and a small pore molecular sieve having a maximum ring size of eight tetrahedral atoms selected from the group of Framework Type Codes consisting of CHA, LEV, ERI and DDR. The chemical process undergoes at least one period of exposure to a reducing atmosphere. The catalyst has an initial activity, and the catalyst has a final activity after the at least one period of exposure to the reducing atmosphere. The final activity is within 10% of the initial activity at a temperature between 250 and 350° C.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood, reference is made to the following drawing by way of illustration only, in which:

FIG. 1 is a graph illustrating NO_(x) conversion of medium pore and large pore molecular sieve supported copper catalysts after lean hydrothermal aging and lean/rich cycle aging;

FIG. 2 is a graph illustrating NO_(x) conversion of Fe/molecular sieve catalysts after lean hydrothermal aging and lean/rich cycle aging;

FIG. 3 is a graph illustrating NO_(x) conversion of small pore molecular sieve supported copper catalysts according to embodiments of the invention and a comparative Cu/Beta catalyst after lean hydrothermal aging and lean/rich cycle aging; and

FIG. 4 is a graph illustrating NO_(x) conversion efficiency over a NAC and combined NAC+SCR systems with different SCR catalysts according to embodiments of the invention and comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

A method of treating NO_(x) in an exhaust gas of a lean burn internal combustion engine is to store the NO_(x) from a lean gas in a basic material and then to release the NO_(x) from the basic material and reduce it periodically using a rich gas. The combination of a basic material (such as an alkali metal, alkaline earth metal or a rare earth metal), and a precious metal (such as platinum), and possibly also a reduction catalyst component (such as rhodium) is typically referred to as a NO_(x) adsorber catalyst (NAC), a lean NO_(x) trap (LNT), or a NO_(x) storage/reduction catalyst (NSRC). As used herein, NO_(x) storage/reduction catalyst, NO_(x) trap, and NO_(x) adsorber catalyst (or their acronyms) may be used interchangeably.

Under certain conditions, during the periodically rich regeneration events, NH₃ may be generated over a NO_(x) adsorber catalyst. The addition of a SCR catalyst downstream of the NO_(x) adsorber catalyst may improve the overall system NO_(x) reduction efficiency. In the combined system, the SCR catalyst is capable of storing the released NH₃ from the NAC catalyst during rich regeneration events and utilizes the stored NH₃ to selectively reduce some or all of the NO_(x) that slips through the NAC catalyst during the normal lean operation conditions. As used herein, such combined systems may be shown as a combination of their respective acronyms, NAC+SCR or LNT+SCR.

The combined NAC+SCR system imposes additional requirements on the SCR catalyst component. Namely, besides having good activity and excellent thermal stability, the SCR catalyst has to be stable against lean/rich excursions. Such lean/rich excursions not only may occur during the regular NAC regeneration events, but also may happen during the NAC desulfation events. During the NAC desulfation events, the SCR catalyst may be exposed to temperatures much higher than it would be exposed to during the regular NO_(x) regeneration events. Therefore, a good SCR catalyst that is suitable for the NAC+SCR systems needs to be durable after being exposed to a reducing atmosphere at high temperature. Although the present invention is described herein with particular emphasis on the SCR embodiment, it is contemplated that the present invention may encompass any catalysts which lose activity when exposed to a reducing atmosphere.

Catalysts are often unstable when exposed to a reducing atmosphere, more particularly a high temperature reducing atmosphere. For example, copper catalysts are unstable during repeated lean/rich high temperature excursions, e.g., as is often encountered in vehicle exhaust gas or an exhaust gas treatment system. The reducing atmosphere occurs in the rich phase of a lean/rich excursion cycle. The reducing atmosphere conditions, however, can occur in a variety of environments including but not limited to environments typical for the regeneration or the desulfation of a NO_(x) adsorber catalyst, and for the active regeneration of a catalyzed soot filter, etc. As used herein, a reducing atmosphere is an atmosphere having a lambda value (air/fuel ratio) of less than 1, i.e., the redox composition is net reducing. Contrastingly, a lean atmosphere is one having a lambda value of greater than 1, i.e., the redox composition is net oxidizing.

Without wishing to be bound to a particular theory, it was believed prior to discovery of the present invention that molecular sieve supported copper catalysts would not maintain stability or activity when exposed to a reducing atmosphere (especially a reducing atmosphere encountered in a repeated lean/rich cycle excursions) because when exposed to the reducing atmosphere, the copper catalysts lost their activity. This loss of activity was suspected to be due to copper migration, sintering, and/or reduced copper dispersion. Surprisingly, we discovered in the present invention that small pore molecular sieve-supported copper catalysts maintained their catalytic activity even though the medium and large pore molecular sieve supported copper catalysts could not. It is believed that small pore molecular sieves provide a restriction on the copper from migrating out of the framework, sintering, losing copper dispersion, and beneficially resulting in an improved stability and activity of the catalyst. The medium and large pore molecular sieves, however, do not maintain their stability and activity when exposed to a reducing atmosphere possibly because of the effects of copper migration, sintering, and/or reduced copper dispersion.

According to one embodiment of the present invention, a method of using a catalyst comprises exposing a catalyst to at least one reactant in a chemical process. The catalyst comprises copper and a small pore molecular sieve having a maximum ring size of eight tetrahedral atoms. The chemical process undergoes at least one period of exposure to a reducing atmosphere. The catalyst has an initial activity and the catalyst has a final activity after the at least one period of exposure to the reducing atmosphere. The final activity is within 30% of the initial activity at a temperature between 200 and 500° C.

A method of using a catalyst comprises exposing a catalyst to at least one reactant in a chemical process. As used herein, chemical process can include any suitable chemical process using a catalyst comprising a small pore molecular sieve comprising copper and encountering reducing conditions. Typical chemical processes include, but are not limited to, exhaust gas treatment such as selective catalytic reduction using nitrogenous reductants, lean NO_(x) catalyst, catalyzed soot filter, or a combination of any one of these with a NO_(x) adsorber catalyst or a three-way catalyst (TWC), e.g., NAC+(downstream)SCR or TWC+(downstream)SCR.

A method of using a catalyst comprises exposing a catalyst to at least one reactant. The reactant may include any reactants typically encountered in the chemical processes above, Reactants may include a selective catalytic reductant, such as ammonia. Selective catalytic reduction may include (1) using ammonia or a nitrogenous reductant or (2) a hydrocarbon reductant (the latter also known as lean NO_(x) catalysis). Other reactants may include nitrogen oxides and oxygen.

The catalyst comprises copper and a small pore molecular sieve having a maximum ring size of eight tetrahedral atoms. As is used herein “molecular sieve” is understood to mean a metastable material containing tiny pores of a precise and uniform size that may be used as an adsorbent for gases or liquids. The molecules which are small enough to pass through the pores are adsorbed while the larger molecules are not. The molecular sieve framework may be defined as is generally acceptable by the International Zeolite Association framework type codes (at http://www.iza-online.org/). These molecular sieves are described in more detail below.

Molecular sieves are typically defined by the member rings as follows: large pore rings are 12-member rings or larger; medium pore rings are 10-member rings; and small pore rings are 8-member rings or smaller. The catalyst in the present invention is a small pore ring having a maximum ring size of eight tetrahedral atoms.

Most catalysts are supported on medium pore (10-ring, such as ZSM-5) or large pore (12-ring, such as Beta) molecular sieves. A molecular sieve supported copper SCR catalyst, for example, may exhibit wide temperature windows under NO only conditions. These catalysts, however, are not stable against repeated lean/rich high temperature aging as is demonstrated in FIG. 1. In FIG. 1, a Cu/Beta catalyst (large pore) and a Cu/ZSM-5 catalyst (medium pore) are shown under hydrothermal aging conditions and lean/rich aging conditions. As is evidenced by the dotted lines representing the lean/rich aging conditions, these types of catalysts are not suitable when exposed to repeated reducing conditions. In particular, these catalysts are not suitable for NAC+SCR applications.

Molecular sieve supported iron SCR catalysts, although not as active as molecular sieve supported copper catalysts at low temperatures (e.g. <350° C.), are stable against repeated lean/rich high temperature aging as shown in FIG. 2. In FIG. 2, Fe/Ferrierite, Fe/ZSM-5, and Fe/Beta are shown after hydrothermal aging and lean/rich aging conditions. Accordingly, molecular sieve supported iron catalysts have been the technology of choice due to their excellent stability against cycled lean/rich aging, e.g., as is encountered in NAC+SCR applications.

Small pore molecular sieve supported Cu catalysts have been demonstrated to exhibit improved NH₃-SCR activity and excellent thermal stability. According to one aspect of the invention, it was found that this type of catalyst also tolerates repeated lean/rich high temperature aging. FIG. 3 compares a series of small pore molecular sieve supported Cu catalysts (Cu/SAPO-34, Cu/Nu-3, and Cu/SSZ-13, respectively) against a comparative large pore catalyst (Cu/Beta) after 700° C./2 hours hydrothermal aging and 600° C./12 hours cycled lean/rich aging, respectively. As is evident in FIG. 3, the catalysts with small pore molecular sieve are very stable against lean/rich aging. In particular, the Cu/SAPO-34 catalyst exhibited exceptionally good low temperature activity and showed no activity degradation after cycled lean/rich aging, i.e., repeated exposure to a reducing atmosphere.

The catalysts in embodiments of the present invention show a much wider temperature window of high NO_(x) conversion. The temperature range of improved conversion efficiency may range from about 200 to 500° C., more particularly from 200 to 450° C., or most significantly from about 200 to 400° C. In these temperature ranges, conversion efficiencies may range from greater than 55% to 100%, more preferably greater than 90% efficiency, and even more preferably greater than 95% efficiency. In particular, combined NAC+SCR systems show a much wider temperature window of high NO_(x) conversion compared to either NAC catalysts alone or NAC+SCR systems using a Fe/molecular sieve SCR catalyst. See FIG. 4. For example at about 250° C. and about 300° C., the NO_(x) conversion efficiencies for systems subjected to lean/rich aging are as follows:

System (undergone lean/rich NO_(X) Conversion % NO_(X) Conversion aging) at 250° C. % at 300° C. NAC alone 73 92 NAC + Fe/Beta SCR catalyst 87 90 NAC + Cu/SSZ-13 SCR catalyst 93 97 NAC + Cu/SAPO-34 SCR 97 96 catalyst

As is evident from these results, the use of the NAC+Cu/small pore molecular sieve catalyst shows dramatic improvement in conversion efficiencies. These improvements are to the final NO_(x) emissions. Thus, an improvement from about 87% NO_(x) conversion (about 13% NO_(x) remaining) to about 97% NO_(x) conversion (about 3% NO_(x) remaining) is about a 433% improvement in efficiency, based on the percent NO_(x) remaining.

The catalyst has an initial activity and the catalyst has a final activity after the at least one period of exposure to the reducing atmosphere. The initial activity may include a baseline aging under hydrothermal conditions. Hydrothermal conditions may include aging at 700° C. for 2 hours with 5% H₂O in air.

The chemical process undergoes at least one period of exposure to a reducing atmosphere. The reducing atmosphere may include any suitable reducing atmosphere such as during rich conditions in a lean/rich aging cycle. For example, a localized reducing atmosphere may also occur during catalyzed soot filter regeneration. The at least one period of exposure may include repeated exposures to reducing conditions or a prolonged exposure to reducing conditions. For example, a repeated exposure may include a cycled lean/rich aging at 600° C. for 12 hours. A lean cycle may last from 15 seconds to several tens of minutes, and a rich cycle may last from less than 1 second to several minutes. For example, the lean portion of the cycle may consist of exposure to 200 ppm NO, 10% O₂, 5% H₂O, 5% CO₂ in N₂, and the rich portion of the cycle may consist of exposure to 200 ppm NO, 5000 ppm C₃H₆, 1.3% H₂, 4% CO, 1% O₂, 5% H₂O, 5% CO₂ in N₂. The reducing atmosphere may be a high temperature reducing atmosphere. A high temperature reducing atmosphere may occur at a temperature from about 150° C. to 850° C. or more particularly from about 450° C. to 850° C.

The final activity is within 30% of the initial activity at a temperature between 200 and 500° C. Preferably, the final activity is within 10% of the initial activity at a temperature between 200 and 500° C. More preferably, the final activity is within 5% of the initial activity at a temperature between 200 and 500° C. Most preferably, the so final activity is within 3% of the initial activity at a temperature between 200 and 500° C. As used herein, when the final activity is given as a percentage of the initial activity, it is given as an average of percentages over the temperature range provided; in other words, if a final activity is said to be within 30% of the initial activity at a temperature between 200 and 500° C., it need not be less than 30% at every temperatures tested in that range, but would merely average less than 30% over the temperatures tested. Moreover, while activity is identified as NO_(x) conversion in the examples of this application, the activity could be some other measure of catalyst activity depending on the chemical process, as is known in the art. The data showing catalyst activity and the percentage of initial activity to final activity is evidenced in the following tables (See also FIG. 3). A negative number means that the activity after exposure to the reducing conditions actually improved relative to the initial activity (and therefore would certainly be “within” a certain positive percentage of the initial activity):

For the embodiment using Cu/Nu-3 the following data was obtained:

TEMP HT Aging TEMP LR Aging % 150 9 150 9 −9% 200 50 198 52 −2% 250 76 250 75 1% 350 72 350 69 4% 450 62 450 58 6% 550 45 550 43 3% 650 27 650 26 2%

Thus, the lean/rich aging % NO_(x) reduction was within about 6% of the hydrothermal aging % NO_(x) reduction. Accordingly, the catalyst remained stable and had good activity following repeated exposure to reducing conditions throughout the temperature ranging from about 150 to about 650° C.

For the embodiment using Cu/SSZ-13 the following data was obtained:

TEMP HT Aging TEMP LR Aging % 164 61 160 19 68% 218 100 216 71 29% 269 100 269 97 3% 373 97 372 86 12% 473 86 474 68 20% 572 64 573 41 36% 668 22 669 −7 134%

Thus, the lean/rich aging % NO_(x) reduction was within about 30% of the hydrothermal aging % NO_(x) reduction throughout the temperature ranging from about 200 to about 500° C.

For the embodiment using Cu/SAPO34 the following data was obtained:

TEMP HT Aging TEMP LR Aging % 156 33 156 47 −41%  211 95 212 97 −3% 264 99 265 99 0.04%   366 89 366 89 −0.01%   464 86 465 83  3% 561 70 564 62 11% 658 34 662 26 22%

Thus, the lean/rich aging % NO_(x) reduction was within about 3% of the hydrothermal aging % NO_(x) reduction throughout the temperature ranging from about 200 to about 500° C., and within about 10% at temperatures ranging from about 200 to 560° C.

As a comparative example a Cu/Beta, large pore molecular sieve catalyst, was compared:

TEMP HT Aging TEMP LR Aging % 158 21 152 9 57% 200 72 199 24 67% 250 93 250 30 67% 350 93 351 26 72% 450 82 450 37 56% 550 82 550 54 35% 650 64 650 49 24%

The Cu/Beta comparative example showed poor activity following lean/rich cycled aging. Thus, exposure to a reducing atmosphere for the copper molecular sieve catalysts causes poor stability and activity as was contemplated prior to discovery of the present invention.

According to one embodiment of the present invention, a method of using a catalyst comprises exposing a catalyst to at least one reactant comprising nitrogen oxides in a chemical process comprising exhaust gas treatment. The catalyst comprises copper and a small pore molecular sieve having a maximum ring size of eight tetrahedral atoms selected from the group of Framework Type Codes consisting of CHA, LEV, ERI and DDR. The chemical process undergoes at least one period of exposure to a reducing atmosphere. The catalyst has an initial activity, and the catalyst has a final activity after the at least one period of exposure to the reducing atmosphere. The final activity is within 10% of the initial activity at a temperature between 250 and 350° C. In a preferred embodiment, the catalyst has a final activity that is within 3% of the initial activity at a temperature between 250 and 350° C.

In an embodiment of the invention, the catalysts have been combined with a NAC (NO_(x) adsorber catalyst) and tested as NAC+SCR systems. FIG. 4 compares the NO_(x) reduction efficiency over a NAC alone and NAC+SCR systems with different SCR small pore molecular sieve catalysts (Cu/SAPO-34, and Cu/SSZ-13), and a comparative example of a Fe/beta catalyst. Combining an Fe/molecular sieve SCR with a NAC catalyst has been shown to improve the system NO_(x) conversion compared to NAC alone. Remarkably, however, the other two systems with a small pore molecular sieve comprising copper, i.e., Cu/SAPO-34 or Cu/SSZ-13, also exhibited further improved NO_(x) removal efficiency. This is especially evident at low temperatures (200-350° C.). These results clearly suggest that small pore molecular sieve supported Cu catalysts offers new potential to further improve the performance of NAC+SCR systems.

In addition to NAC+SCR applications, the small pore molecular sieve supported Cu catalysts offer significant performance advantages for other applications that may be exposed to a high temperature reducing atmosphere. For example, a small pore molecular sieve supported Cu catalyst may be used in a reducing atmosphere which occurs during active regeneration of a SCR/DPF (diesel particulate filter). Small pore molecular sieve supported Cu catalysts provide excellent thermal durability and exceptional stability against reducing conditions, e.g., rich aging that occurs in exhaust gas treatment systems.

It will be appreciated that by defining the molecular sieve by their Framework Type Codes we intend to include the “Type Material” and any and all isotypic framework materials. (The “Type Material” is the species first used to establish the framework type). Reference is made to Table 1, which lists a range of illustrative molecular sieve materials for use in the present invention. For the avoidance of doubt, unless otherwise made clear, reference herein to a molecular sieve by name, e.g. “chabazite”, is to the molecular sieve material per se (in this example the so naturally occurring type material chabazite) and not to any other material designated by the Framework Type Code to which the individual molecular sieve may belong, e.g. some other isotypic framework material. Use of a FTC herein is intended to refer to the Type Material and all isotypic framework materials defined by that FTC.

The distinction between molecular sieve type materials, such as naturally occurring (i.e. mineral) chabazite, and isotypes within the same Framework Type Code is not merely arbitrary, but reflects differences in the properties between the materials, which may in turn lead to differences in activity in the method of the present invention. It will be appreciated, e.g. from Table 1 hereinbelow, that by “MeAPSO” and “MeAlPO” we intend zeotypes substituted with one or more metals. Suitable substituent metals include one or more of, without limitation, As, B, Be, Co, Fe, Ga, Ge, Li, Mg, Mn, Zn and Zr.

In a particular embodiment, the small pore molecular sieve catalysts for use in the present invention can be selected from the group consisting of aluminosilicate molecular sieves, metal-substituted aluminosilicate molecular sieves and aluminophosphate molecular sieves. Aluminophosphate molecular sieves with application in the present invention include aluminophosphate (AlPO) molecular sieves, metal substituted (MeAlPO) molecular sieves, silico-aluminophosphate (SAPO) molecular sieves and metal substituted silico-aluminophosphate (MeAPSO) molecular sieves.

In one embodiment, the small pore molecular sieve is selected from the group of Framework Type Codes consisting of: ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG and ZON.

In an embodiment, the small pore molecular sieve containing a maximum ring size of eight tetrahedral atoms is selected from the group of Framework Type Codes consisting of CHA, LEV, ERI, and DDR. In a preferred embodiment, the small pore molecular sieve comprises a CHA Framework Type Code selected from SAPO-34 or SSZ-13. In an another embedment, the small pore molecular sieve comprises a LEV Framework Type Code Nu-3.

Molecular sieves with application in the present invention can include those that have been treated to improve hydrothermal stability. Illustrative methods of improving hydrothermal stability include:

(i) Dealumination by: steaming and acid extraction using an acid or complexing agent: e.g. (EDTA—ethylenediaminetetracetic acid); treatment with acid and/or complexing agent; treatment with a gaseous stream of SiCl₄ (replaces Al in the molecular sieve framework with Si);

(ii) Cation exchange—use of multi-valent cations such as La; and

(iii) Use of phosphorous containing compounds (see e.g. U.S. Pat. No. 5,958,818).

Illustrative examples of suitable small pore molecular sieves are set out in Table 1.

TABLE 1 Small Pore Molecular Sieve Molecular Sieve Framework Type material* and Type (by illustrative isotypic Framework framework Additional Type Code) structures Dimensionality Pore size (Å) info ACO *ACP-1 3D 3.5 × 2.8, 3.5 × Ring sizes - 3.5 8, 4 AEI *AIPO-18 3D 3.8 × 3.8 Ring sizes - 8, 6, 4 [Co—Al—P—O]-AEI SAPO-18 SIZ-8 SSZ-39 AEN *AIPO-EN3 2D 4.3 × 3.1, 2.7 × Ring sizes - 5.0 8, 6, 4 AIPO-53(A) AIPO-53(B) [Ga—P—O]-AEN CFSAPO-1A CoIST-2 IST-2 JDF-2 MCS-1 MnAPO-14 Mu-10 UiO-12-500 UiO-12-as AFN *AIPO-14 3D 1.9 × 4,6, 2.1 × Ring sizes - 4.9, 3.3 × 4.0 8, 6, 4 |(C₃N₂H₁₂)—|[Mn—Al—P—O]- AFN GaPO-14 AFT *AIPO-52 3D 3.8 × 3.2, 3.8 × Ring sizes - 3.6 8, 6, 4 AFX *SAPO-56 3D 3.4 × 3.6 Ring sizes - 8, 6, 4 MAPSO-56, M = Co, Mn, Zr SSZ-16 ANA *Analcime 3D 4.2 × 1.6 Ring sizes - 8, 6, 4 AIPO₄-pollucite AIPO-24 Ammonioleucite [Al—Co—P—O]-ANA [Al—Si—P—O]-ANA |Cs—|[Al—Ge—O]-ANA |Cs—|[Be—Si—O]-ANA |Cs₁₆|[Cu₈Si₄₀O₉₆]- ANA |Cs—Fe|[Si—O]-ANA |Cs—Na—(H₂O)|[Ga—Si—O]- ANA [Ga—Ge—O]-ANA |K—|[B—Si—O]-ANA |K—|[Be—B—P—O]-ANA |Li—|[Li—Zn—Si—O]-ANA |Li—Na|[Al—Si—O]-ANA |Na—|[Be—B—P—O]- ANA |(NH₄)—|[Be—B—P—O]- ANA |(NH₄)—|[Zn—Ga—P—O]- ANA [Zn—As—O]-ANA Ca-D Hsianghualite Leucite Na—B Pollucite Wairakite APC *AIPO-C 2D 3.7 × 3.4, 4.7 × Ring sizes - 2.0 8, 6, 4 AIPO-H3 CoAPO-H3 APD *AIPO-D 2D 6.0 × 2.3, 5.8 × Ring sizes - 1.3 8, 6, 4 APO-CJ3 ATT *AIPO-12-TAMU 2D 4.6 × 4.2, 3.8 × Ring sizes - 3.8 8, 6, 4 AIPO-33 RMA-3 CDO *CDS-1 2D 4.7 × 3.1, 4.2 × Ring sizes - 2.5 8, 5 MCM-65 UZM-25 CHA *Chabazite 3D 3.8 × 3.8 Ring sizes - 8, 6, 4 AIPO-34 [Al—As—O]-CHA [Al—Co—P—O]-CHA |Co|[Be—P—O]-CHA |Co₃(C₆N₄H₂₄)₃(H₂O)₉| [Be₁₈P₁₈O₇₂]- CHA [Co—Al—P—O]-CHA |Li—Na|[Al—Si—O]- CHA [Mg—Al—P—O]-CHA [Si—O]-CHA [Zn—Al—P—O]-CHA [Zn—As—O]-CHA CoAPO-44 CoAPO-47 DAF-5 GaPO-34 K-Chabazite Linde D Linde R LZ-218 MeAPO-47 MeAPSO-47 (Ni(deta)₂)-UT-6 Phi SAPO-34 SAPO-47 SSZ-13 UiO-21 Willhendersonite ZK-14 ZYT-6 DDR *Deca-dodecasil 3R 2D 4.4 × 3.6 Ring sizes - 8, 6, 5, 4 [B—Si—O]-DDR Sigma-1 ZSM-58 DFT *DAF-2 3D 4.1 × 4.1, 4.7 × Ring sizes - 1.8 8, 6, 4 ACP-3, [Co—Al—P—O]- DFT [Fe—Zn—P—O]-DFT [Zn—Co—P—O]-DFT UCSB-3GaGe UCSB-3ZnAs UiO-20, [Mg—P—O]- DFT EAB *TMA-E 2D 5.1 × 3.7 Ring sizes - 8, 6, 4 Bellbergite EDI *Edingtonite 3D 2.8 × 3.8, 3.1 × Ring sizes - 2.0 8, 4 |(C₃H₁₂N₂)_(2.5)| [Zn₅P₅O₂₀]-EDI [Co—Al—P—O]-EDI [Co—Ga—P—O]-EDI |Li—|[Al—Si—O]-EDI |Rb₇ Na (H₂O)₃| [Ga₈Si₁₂O₄₀]-EDI [Zn—As—O]-EDI K—F Linde F Zeoiite N EPI *Epistilbite 2D 4.5 × 3.7, 3.6 × Ring sizes - 3.6 8, 4 ERI *Erionite 3D 3.6 × 5.1 Ring sizes - 8, 6, 4 AIPO-17 Linde T LZ-220 SAPO-17 ZSM-34 GIS *Gismondine 3D 4.5 × 3.1, 4.8 × Ring sizes - 2.8 8, 4 Amicite [Al—Co—P—O]-GIS [Al—Ge—O]-GIS [Al—P—O]-GIS [Be—P—O]-GIS |(C₃H₁₂N₂)₄| [Be₈P₈O₃₂]-GIS |(C₃H₁₂N₂)₄| [Zn₈P₈O₃₂]-GIS [Co—Al—P—O]-GIS [Co—Ga—P—O]-GIS [Co—P—O]-GIS |Cs₄|[Zn₄B₄P₈O₃₂]- GIS [Ga—Si—O]-GIS [Mg—Al—P—O]-GIS |(NH₄)₄|[Zn₄B₄P₈O₃₂]- GIS |Rb₄|[Zn₄B₄P₈O₃₂]- GIS [Zn—Al—As—O]-GIS [Zn—Co—B—P—O]-GIS [Zn—Ga—As—O]-GIS [Zn—Ga—P—O]-GIS Garronite Gobbinsite MAPO-43 MAPSO-43 Na-P1 Na-P2 SAPO-43 TMA-gismondine GOO *Goosecreekite 3D 2.8 × 4.0, 2.7 × Ring sizes - 4.1, 4.7 × 2.9 8, 6, 4 IHW *ITQ-32 2D 3.5 × 4.3 Ring sizes - 8, 6, 5, 4 ITE *ITQ-3 2D 4.3 × 3.8, 2.7 × Ring sizes - 5.8 8, 6, 5, 4 Mu-14 SSZ-36 ITW *ITQ-12 2D 5.4 × 2.4, 3.9 × Ring sizes - 4.2 8, 6, 5, 4 LEV *Levyne 2D 3.6 × 4.8 Ring sizes - 8, 6, 4 AIPO-35 CoDAF-4 LZ-132 NU-3 RUB-1 [B—Si—O]-LEV SAPO-35 ZK-20 ZnAPO-35 KFI ZK-5 3D 3.9 × 3.9 Ring sizes - 8, 6, 4 |18-crown-6|[Al—Si—O]- KFI |Zn—Ga—As—O]-KFI (Cs,K)-ZK-5 P Q MER *Merlinoite 3D 3.5 × 3.1, 3.6 × Ring sizes - 2.7, 5.1 × 3.4, 8, 4 3.3 × 3.3 [Al—Co—P—O]-MER |Ba—|[Al—Si—O]-MER |Ba—Cl—|[Al—Si—O]- MER [Ga—Al—Si—O]-MER [K—|[Al—Si—O]-MER |NH₄—|[Be—P—O]-MER K-M Linde W Zeolite W MON *Montesommaite 2D 4.4 × 3.2, 3.6 × Ring sizes - 3.6 8, 5, 4 [Al—Ge—O]-MON NSI *Nu-6(2) 2D 2.6 × 4.5, 2.4 × Ring sizes - 4.8 8, 6, 5 EU-20 OWE *UiO-28 2D 4.0 × 3.5, 4.8 × Ring sizes - 3.2 8, 6, 4 ACP-2 PAU *Paulingite 3D 3.6 × 3.6 Ring sizes - 8, 6, 4 [Ga—Si—O]-PAU ECR-18 PHI *Phillipsite 3D 3.8 × 3.8, 3.0 × Ring sizes - 4.3, 3.3 × 3.2 8, 4 [Al—Co—P—O]-PHI DAF-8 Harmotome Wellsite ZK-19 RHO *Rho 3D 3.6 × 3.6 Ring sizes - 8, 6, 4 [Be—As—O]-RHO [Be—P—O]-RHO [Co—Al—P—O]-RHO |H—|[Al—Si—O]-RHO [Mg—Al—P—O]-RHO [Mn—Al—P—O]-RHO |Na₁₆ Cs₈| [Al₂₄Ge₂₄O₉₆]-RHO |NH₄—|[Al—Si—O]-RHO |Rb—|[Be—As—O]-RHO Gallosilicate ECR-10 LZ-214 Pahasapaite RTH *RUB-13 2D 4.1 × 18, 5.6 × Ring sizes - 2.5 8, 6, 5, 4 SSZ-36 SSZ-50 SAT *STA-2 3D 5.5 × 3.0 Ring sizes - 8, 6, 4 SAV *Mg-STA-7 3D 3.8 × 3.8, 3.9 × Ring sizes - 3.9 8, 6, 4 Co-STA-7 Zn-STA-7 SBN *UCSB-9 3D TBC Ring sizes - 8, 4, 3 SU-46 SIV *SIZ-7 3D 3.5 × 3.9, 3.7 × Ring sizes - 3.8, 3.8 × 3.9 8, 4 THO *Thomsonite 3D 2.3 × 3.9, 4.0 × Ring sizes - 2.2, 3.0 × 2.2 8, 4 [Al—Co—P—O]-THO [Ga—Co—P—O]-THO |Rb₂₀|[Ga₂₀Ge₂₀O₈₀]- THO [Zn—Al—As—O]-THO [Zn—P—O]-THO [Ga—Si—O]-THO) [Zn—Co—P—O]-THO TSC *Tschortnerite 3D 4.2 × 4.2, 5.6 × Ring sizes - 3.1 8, 6, 4 UEI *Mu-18 2D 3.5 × 4.6, 3.6 × Ring sizes - 2.5 8, 6, 4 UFI *UZM-5 2D 3.6 × 4.4, 3.2 × Ring sizes - 3.2 (cage) 8, 6, 4 VNI *VPI-9 3D 3.5 × 3.6, 3.1 × Ring sizes - 4.0 8, 5, 4, 3 YUG *Yugawaralite 2D 2.8 × 3.6, 3.1 × Ring sizes - 5.0 8, 5, 4 Sr-Q ZON *ZAPO-M1 2D 2.5 × 5.1, 3.7 × Ring sizes - 4.4 8, 6, 4 GaPO-DAB-2 UiO-7

Small pore molecular sieves with particular application for exposure to reducing conditions are set out in Table 2.

TABLE 2 Preferred Small Pore Molecular Sieves. Structure Molecular Sieve CHA SAPO-34 AIPO-34 SSZ-13 LEV Levynite Nu-3 LZ-132 SAPO-35 ZK-20 ERI Erionite ZSM-34 Linde type T DDR Deca-dodecasil 3R Sigma-1 KFI ZK-5 18-crown-6 [Zn—Ga—As—O]-KFI EAB TMA-E PAU ECR-18 MER Merlinoite AEI SSZ-39 GOO Goosecreekite YUG Yugawaralite GIS P1 VNI VPI-9

Molecular sieves for use in the present application include natural and synthetic molecular sieves, preferably synthetic molecular sieves because the molecular sieves can have a more uniform: silica-to-alumina ratio (SAR), crystallite size, crystallite morphology, and the absence of impurities (e.g. alkaline earth metals). Small pore aluminosilicate molecular sieves may have a silica-to-alumina ratio (SAR) of from 2 to 300, optionally 4 to 200, and preferably 8 to 150. It will be appreciated that higher SAR ratios are preferred to improve thermal stability but this may negatively affect transition metal exchange.

The at least one reactant may contact the catalyst at a gas hourly space velocity of from 5,000 hr⁻¹ to 500,000 hr⁻¹, optionally from 10,000 hr⁻¹ to 200,000 hr⁻¹.

Small pore molecular sieves for use in the invention may have three-dimensional dimensionality, i.e. a pore structure which is interconnected in all three crystallographic dimensions, or two-dimensional dimensionality. In one embodiment, the small pore molecular sieves for use in the present invention consist of molecular sieves having three-dimensional dimensionality. In another embodiment, the small pore molecular sieves for use in the present invention consist of molecular sieves having two-dimensional dimensionality.

The total of the copper metal that can be included in the molecular sieve can be from 0.01 to 20 wt %, based on the total weight of the catalyst. In one embodiment, the total of the copper that can be included can be from 0.1 to 10 wt %. In a particular embodiment, the total of copper that can be included is from 0.5 to 5 wt %. The copper may be included in the molecular sieve by any feasible method. For example, it can be added after the molecular sieve has been synthesized, e.g. by incipient wetness or exchange process; or can be added during molecular sieve synthesis.

A preferred two dimensional small pore molecular sieve for use in the present invention consists of Cu/LEV, such as Cu/Nu-3, whereas a preferred copper-containing three dimensional small pore molecular sieve/aluminophosphate molecular sieve for use in the present invention consists of Cu/CHA, such as Cu/SAPO-34 or Cu/SSZ-13.

The molecular sieve catalysts for use in the present invention can be coated on a suitable substrate monolith or can be formed as extruded-type catalysts, but are preferably used in a catalyst coating. In one embodiment, the molecular sieve catalyst is coated on a flow-through monolith substrate (i.e. a honeycomb monolithic catalyst support structure with many small, parallel channels running axially through the entire part) or filter monolith substrate such as a wall-flow filter etc. The molecular sieve catalyst for use in the present invention can be coated, e.g. as a washcoat component, on a suitable monolith substrate, such as a metal or ceramic flow through monolith substrate or a filtering substrate, such as a wall-flow filter or sintered metal or partial filter (such as is disclosed in WO 01/80978 or EP 1057519, the latter document describing a substrate comprising convoluted flow paths that at least slows the passage of soot therethrough). Alternatively, the molecular sieves for use in the present invention can be synthesized directly onto the substrate. Alternatively, the molecular sieve catalysts according to the invention can be formed into an extruded-type flow through catalyst.

Washcoat compositions containing the molecular sieves for use in the present invention for coating onto the monolith substrate for manufacturing extruded type substrate monoliths can comprise a binder selected from the group consisting of alumina, silica, (non molecular sieve) silica-alumina, naturally occurring clays, TiO₂, ZrO₂, and SnO₂.

In one embodiment, the at least one reactant, e.g., nitrogen oxides, are reduced with the reducing agent at a temperature of at least 100° C. In another embodiment, the at least one reactant are reduced with the reducing agent at a temperature from about 150° C. to 750° C. In a particular embodiment, the temperature range is from 175 to 550° C., or more particularly from 175 to 400° C.

For a reactant including nitrogen oxides, the reduction of nitrogen oxides may be carried out in the presence of oxygen or in the absence of oxygen. The source of nitrogenous reductant can be ammonia per se, hydrazine or any suitable ammonia precursor (such as urea ((NH₂)₂CO)), ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate or ammonium formate.

The method may be performed on a gas derived from a combustion process, such as from an internal combustion engine (whether mobile or stationary), a gas turbine and coal or oil fired power plants. The method may also be used to treat gas from industrial processes such as refining, from refinery heaters and boilers, furnaces, the chemical processing industry, coke ovens, municipal waste plants and incinerators, coffee roasting plants, etc.

In a particular embodiment, the method is used for treating exhaust gas from a vehicular internal combustion engine with a lean/rich cycle, such as a diesel engine, a gasoline engine, or an engine powered by liquid petroleum gas or natural gas.

For a reactant including nitrogen oxides, the nitrogenous reductant may be metered into the flowing exhaust gas only when it is determined that the molecular sieves catalyst is capable of catalyzing NO_(x) reduction at or above a desired efficiency, such as at above 100° C., above 150° C. or above 175° C. The determination by the control means can be assisted by one or more suitable sensor inputs indicative of a condition of the engine selected from the group consisting of: exhaust gas temperature, catalyst bed temperature, accelerator position, mass flow of exhaust gas in the system, manifold vacuum, ignition timing, engine speed, lambda value of the exhaust gas, the quantity of fuel injected in the engine, the position of the exhaust gas recirculation (EGR) valve and thereby the amount of EGR and boost pressure.

Metering may be controlled in response to the quantity of nitrogen oxides in the exhaust gas determined either directly (using a suitable NO_(x) sensor) or indirectly, such as using pre-correlated look-up tables or maps—stored in the control means—correlating any one or more of the abovementioned inputs indicative of a condition of the engine with predicted NO_(x) content of the exhaust gas.

The entire contents of any and all patents and references cited herein are incorporated herein by reference.

EXAMPLES

Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention,

1. Steady State SCR Evaluation

Steady-state selective catalytic reduction (SCR) activity tests were conducted in a quartz reactor of 24 inches in length, and uniformly heated by two tube furnace units of 12 inches length. Experiments were performed at a gas hourly space velocity of 30,000 hr⁻¹ utilizing catalyst dimensions of 1 inch diameter×1 inch length. All gas lines directly connected to the reactor were maintained at 130° C. by heating tape to prevent gas species adsorption on the walls of the gas line. Water vapor was provided by a water bomb, which was constantly maintained at 70° C.

Prior to reaching the catalyst bed, the feed gas was heated and mixed upstream in the reactor via an inert thermal mass. The temperature of the gas stream was monitored at the catalyst inlet, center of the catalyst bed, and at the outlet by k-type thermocouples. Reacted feed gas was analyzed by a FTIR downstream from the catalyst bed at a sampling rate of 1.25 s⁻¹. The composition of the inlet feed gas could be determined by sampling from a bypass valve located upstream of the reactor.

Steady-state SCR experiments were initially performed on catalyst samples that had been hydrothermally (HT) aged at 700° C. for 2 hours in the presence of air containing 4.5% H₂O. All steady state experiments were conducted using a feed gas of NO and NH₃ containing 350 ppm of NO, with an ammonia-to-NO (ANR) ratio of 1 (i.e., 350 ppm of NH₃). The remainder of the feed gas composition was as follows: 14% O₂, 4.6% H₂O, 5% CO₂, balance N₂. The steady-state NO_(x) conversion was determined at catalyst bed temperatures of 150° C., 200° C., 250° C., 350° C., 450° C., 550° C., and 650° C.

Catalysts were then aged under lean-rich cycling conditions at 600° C. for 12 h. The lean portion of the cycle consisted of exposure to 200 ppm NO, 10% O₂, 5% H₂O, 5% CO₂ in N₂ for 5 seconds at a space velocity of 30,000 h⁻¹. The rich portion of the cycle consisted of exposure to 200 ppm NO, 5000 ppm C₃H₆, 1.3% H₂, 4% CO, 1% O₂, 5% H₂O, 5% CO₂ in N₂ for 15 seconds. After the aging, steady-state SCR experiments were performed as described above.

In FIG. 3, the NO_(x) conversion efficiency is shown for embodiments of the present invention and a comparative example. Cu/SAPO-34, Cu/Nu-3 and Cu/SSZ-13, small pore molecular sieve catalysts according to embodiments of the invention, are shown after having undergone the above described hydrothermal aging treatment and lean/rich aging treatment, respectively. A comparative example showing Cu/Beta, a large pore molecular sieve catalyst, is also shown after both hydrothermal aging and lean/rich aging. As is evident, the small pore molecular sieve catalysts demonstrated enhanced NO_(x) conversion efficiencies especially in the temperature so window of 200 to 500° C.

2. NAC+SCR Experiments

NO_(x) adsorber catalyst (NAC) and SCR cores were initially hydrothermally aged at 750° C. for 16 hours in a 4.5% H₂O in air gas mixture. Samples were then mounted in the same reactor setup described above with the NAC catalyst mounted directly in front of the SCR catalyst. The catalysts were aged at 600° C. for 12 hours under the lean-rich aging conditions described above (5 seconds lean/15 seconds rich).

The catalysts were then cooled to 450° C. under lean-rich cycling (60 seconds lean/5 seconds rich, same gas compositions). At 450° C., 25 lean-rich cycles were completed (60 seconds lean/5 seconds rich) with the last five cycles used to determine an average cycle NO_(x) conversion for the catalyst. After the 25^(th) cycle, the catalyst was held under the lean gas composition for 5 minutes. The catalyst was then cooled and evaluated at 400° C., 350° C., 300° C., 250° C., 200° C., and 175° C. following the aforementioned cycle procedure.

In FIG. 4, the NO_(x) conversion efficiency is shown for embodiments of the present invention and two comparative examples. NAC+Cu/SAPO-34 and NAC+Cu/SSZ-13, small pore molecular sieve catalysts according to embodiments of the invention are shown after having undergone the above described lean/rich aging treatment. A comparative example showing NAC alone and NAC+Fe/Beta, a large pore molecular sieve catalyst, is also shown after lean/rich aging. As is evident, the small pore molecular sieve catalysts demonstrate enhanced NO_(x) conversion efficiencies comparable to and/or better than NAC alone or NAC+Fe/Beta, especially in the temperature window of 250 to 450° C.

While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention. 

1-22. (canceled)
 23. A method for using a catalyst comprising: a. exposing an SCR catalyst comprising copper and a small-pore molecular sieve having an AFX structure to a reducing atmosphere comprising NH₃ generated over a NO_(x) adsorber catalyst and using the SCR catalyst to store NH₃ during the exposing step, and b. contacting the SCR catalyst with NO_(x) in a non-reducing atmosphere to effect selective reduction of said NO_(x) by the stored NH₃.
 24. The method of claim 23, wherein the small pore molecular sieve comprises SAPO-56 or SSZ-16.
 25. The method of claim 23, wherein said small pore molecular sieve is SSZ-16 and has a silica-to-alumina ratio of 8 to
 150. 26. A system for treating exhaust gas comprising a NO_(x) adsorber catalyst disposed upstream of an SCR catalyst, wherein said SCR catalyst comprises copper and small-pore molecular sieve having an AFX structure.
 27. The system of claim 26, wherein the small pore molecular sieve comprises SAPO-56 or SSZ-16.
 28. The system of claim 26, wherein the small pore molecular sieve is SSZ-16 and has a silica-to-alumina ratio of about 4 to about
 200. 29. The system of claim 26, wherein said copper is present in said small pore molecular sieve in an amount from 0.5 to 5 weight percent.
 30. The system of claim 26, wherein said NO_(x) adsorber catalyst comprises a precious metal and a basic material selected from the group consisting of an alkali metal, an alkaline earth metal, and a rare earth metal.
 31. The system of claim 26, further comprising an ammonia component, wherein said ammonia component is generated by said NO_(x) adsorber catalyst.
 32. The system of claim 26, further comprising an alternating reducing atmosphere and an oxidizing atmosphere, wherein said SCR catalyst is exposed to said alternating atmospheres.
 33. The system of claim 32, wherein said reducing atmosphere is a lean exhaust gas and said oxidizing atmosphere is a rich exhaust gas.
 34. The system of claim 26, wherein said NO_(x) adsorber catalyst is loaded on a first substrate and said SCR catalyst is loaded on a second substrate and the first substrate is disposed in front of the second substrate with respect to the normal flow of exhaust gas through the system. 